FSX Northrop YF17. The Northrop YF-17 (popularly known as Cobra) was a prototype light and daytime fighter aircraft designed for the United States Air Force Light Weight Fighter (LWF) technology assessment software. The LWF project was created because the F-15 Eagle was too large and expensive for many combat tasks. Original aircraft design by JR Lucariny Adaptation to FSX, panels and gauges by Philippe Wallaert